Piercing machines are used in manufacturing seamless steel pipes in the Mannesmann process. A piecing machine includes a pair of skew rolls and a plug. The plug is disposed between the pair of the skew rolls, and is located on a pass line. The piercing machine pushes and squeezes a billet over the plug while rotating the billet in the circumferential direction with the skew rolls, so as to piercing-roll the billet into a hollow shell.
The piercing machine piercing-rolls a billet heated at a high temperature. Hence, the plug over which the billet is pushed and squeezed is subjected to a high temperature and a high pressure. Consequently, melting loss and scoring are likely to be caused to the plug.
In general, oxide scale is formed on the surface of a plug base metal. Such an oxide scale blocks heat from the billet so as to reduce generation of the melting loss. The oxide scale also reduces generation of the scoring.
Unfortunately, the oxide scale is gradually reduced every time the billet is piercing-rolled. If the oxide scale is exhausted, the temperature of the plug base metal begins to increase, which causes the melting loss to the plug.
In order to enhance the usage count of the plug, it has been proposed not only to form a scale on the surface of the plug base metal, but also to adjust a chemical composition of the base metal (see JP4-8498B, JP4-74848A, JP4-270003A, and JP64-7147B, for example).
In order to enhance the usage count of the plug, it has been proposed to form a coating other than the scale on the surface of the plug base metal (see JP10-180315A, and JP4279350B, for example).